Human body will recognize the extraneous matter and start a series of defending process. This defense system is named as immune system. There are many different cells such as leukocytes and lymphocyte, and different protein factors such as immunoglobulins and cytokines working coordinately to protect the body. The immune systems are traditionally divided into innate and adaptive immune systems. Innate immune system is including soluble complement system, polymorphonuclear neutrophils, macrophages and natural killer cells. Adaptive immune system is including humoral and cellular immunity. Humoral immunity as well as cellular immunity involves lymphocyte, lymphokine and immunological memory system. The long-lasting immune memory mounts quick and strong immune responses towards the same pathogen which has invaded the body.
Immune system may respond to different pathogens due to the diversity of major histocompatibility complex (MHC) molecules. The endogenous and exogenous antigens derived from pathogens, are assembled with MHC molecules on the surface of antigen-presenting cells (APC) and then presented to T cells expressing corresponding T cell receptors. MHC in the human beings can be called Human Leukocyte Antigen, HLA, which can be categorized into class I, class II, and class III. HLA class I is widely expressed on all the somatic cells but Class II distribution is restricted to macrophages, B cells and dendritic cells.
Dendritic cells (DC), which have the broadest range of antigen presentation, are professional APC, and named by the appearance of dendrites extending from the cell body. DCs reside in the periphery of body as immature DCs (imDCs). Once pathogen invades human bodies, imDCs capture pathogen-derived antigens, migrate to draining lymph nodes to become mature DCs (mDCs), and present antigens to corresponding T cells there. Therefore, dendritic cells are the starter of the pathogen-specific cellular immune responses.
Natural killer (NK) cells, a key player of innate immune system, spontaneously kill tumor or virally infected cells prior to activation. Mechanisms underlying cytotoxicity of NK cells are grouped into two parts: a) interaction of cell surface tumor necrosis factor superfamily members and their receptors which leads to apoptosis of target cells, (b) release of soluble perforin and granzymes. NK cells are rich with small granules in their cytoplasm contain special proteins such as perforin and proteases known as granzymes. Upon release in close proximity to a cell slated for killing, perforin forms pores in the cell membrane of the target cell through which the granzymes and associated molecules can diffuse in, leading to destruction of target cells. Once virally infected cells or tumor cells have been killed, viral genomic content (CpG or poly I:C), cellular metabolites, and bystander cytokines such as IFN-γ, IL-12 and TNF-α would further activate and augment NK cell activity in term of cytotoxicity and effector cytokine production. Therefore NK cells serve as key innate effector cells targeting to virally infected cells and tumor cells in a non-antigen specific manner while DCs in adaptive immune system trigger antigen-specific cytotoxic T cells which can further clear the infection. Patients deficient in NK cells are proved to be highly susceptible to early phases of herpes virus infection. Interferon-producing killer dendritic cells (IKDCs), a recently identified leukocyte population in mice, express phenotypes of non-T (CD3−), non-B (CD19−), intermediate levels of CD11c, and high levels of B220 and NK-specific markers, including NK1.1, DX5, NKG2D and Ly49 family receptors. IKDCs functionally resemble NK cells in cytotoxicity against tumor cells and in production of abundant IFN-γ. On the other hand, upon stimulation with CpG or tumor cells, IKDCs down-regulate NKG2D, up-regulate MHC II, and acquire moderate APC-like activity that activates antigen-specific T cells. Despite acquisition of APC activity after certain stimulations, IKDCs appear to belong to the NK lineage rather than DC lineage. IKDCs express NK-specific Ner-1 transcripts (encoding NKp46) but not PU.1 that is predominantly expressed in DCs and plasmacytoid DCs. Furthermore, IKDC development parallels NK cells in their strict dependence on the IL-15 cytokine system. Therefore, the putative IKDCs are functionally and developmentally similar to NK cells. Although debates regarding tumoricidal activity and cell lineage development of IKDC were raised herein, further investigations were limited by rare abundance of IKDC in periphery. The frequency of IKDCs in a mouse spleen is below 0.01%, and is even lower in the lymph nodes. Therefore, cumbersome procedure is required for the purification of IKDCs, and the yield is low. This problem has limited the use of IKDCs in research and in application.